I think your vet should be able to write you a prescription and then you can buy the product online or from a mail-order catalog.

I would not suggest using 1800petmeds (even though their prices are cheap) because the manufacturers will not guarantee products sold through them. A more reputable company is Drs. Foster & Smith. I don't buy pet pharmaceuticals online, so I would suggest that you price shop a little, and maybe ask your vet for a suggestion.

Sentinel does not kill adult fleas....but it sterilizes any flea that has a blood meal on my dog. This type of method is referred to as an environmental control. It does not guarantee no fleas, but it means that the population will not get out of hand quickly. Then if I do see fleas, I address them (usually with Capstar or a topical). We have not had a lot of flea problems and so I prefer to err on the side of using less chemicals on my animals.

For more information on Advantage Multi I would suggest searching the internet. You can read stuff from Bayer as well as other third parties. I'm not sure how many vet journals are free to read online...but that might be another way to learn more. Advantage Multi is new to the U.S. (as you mentioned), and I'm sure it has been heavily covered in journals as of late.

Hope this helps.